The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Generally, an internal combustion engine generates a power by taking fuel and air into a combustion chamber and burning the air-fuel mixture. When the air is sucked, intake valves are actuated by driving a camshaft, and the air is sucked into the combustion chamber while the intake valves are open. Further, exhaust valves are actuated by driving the camshaft, and exhaust gas is discharged from the combustion chamber while the exhaust valves are open.
However, improved operation of the intake valve and the exhaust valve depend in part on an engine rotation speed. That is, an appropriate valve lift or valve opening and closing timings is (are) changed according to the engine rotation speed. In order to realize an appropriate valve operation in accordance with the engine rotation speed, a plurality of cam shapes for actuating the valves are designed, or a continuously variable valve lift (CVVL) apparatus that implements the valves to operate with different lifts according to the engine rotation speed has been developed.
Further, a continuous variable valve timing (CVVT) technology for adjusting opening times of the valves has been developed, and the CVVT technology is a technique in which the valve opening and closing timings are changed simultaneously with a valve duration being fixed.
In recent years, there has been developed a technique for controlling a duration for which the valves are open (i.e., a valve duration) based on a driving condition of a vehicle, and the technique for controlling the valve duration has been applied to the vehicle.
On the other hand, the vehicle is equipped with a catalytic converter to reduce emission (EM) contained in the exhaust gas. The exhaust gas flowing out from an engine through an exhaust manifold is driven into a catalytic converter mounted at an exhaust pipe and is purified therein. After that, noise of the exhaust gas is decreased while passing through a muffler and then the exhaust gas is emitted into the air through a tail pipe. The catalytic converter purifies the EM contained in the exhaust gas. In addition, a particulate filter for trapping particulate matter (PM) in the exhaust gas is mounted in the exhaust pipe.
A three-way catalyst (TWC) is one type of the catalytic converter and reacts with hydrocarbon (HC) compounds, carbon monoxide (CO) and nitrogen oxides (NOx), which are harmful components of the exhaust gas, to remove these compounds. The TWCs are mainly installed in gasoline vehicles, and Pt/Rh, Pd/Rh or Pt/Pd/Rh systems are used as the TWCs.
The internal combustion engine equipped with the TWC performs a fuel cut-off to improve fuel economy under specific driving conditions. In this case, an oxygen storage material in the TWC stores oxygen (O2) therein. If the fuel cut-off is terminated (i.e., fuel cut-in) in a state that the oxygen storage material stores a large amount of the O2, the TWC cannot reduce but exhaust the NOx contained in the exhaust gas due to the O2 stored in the oxygen storage material. Therefore, air-fuel ratio (AFR) is made to be rich to remove the 02 stored in the oxygen storage material when performing fuel cut-in after the fuel cut-off. This is called ‘oxygen purge’.
However, since the AFR in the TWC is lean even during the oxygen purge, the TWC cannot reduce but exhaust the NOx. Therefore, it is desirable to reduce an amount of the NOx generated in the engine during the oxygen purge.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the disclosure and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.